CHM 585 Chapter 22

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CHM 585Chapter 22

• Nuclear

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Radioactive

• When nuclei change spontaneously, emitting
  radiation, they are said to be radioactive.
• Radioactive elements are widely used in medicine
  as diagnostic tools and as a means of treatment,
  especially for cancer

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Nuclear Power

• Accounts for about 20 of the total electricity
  generated in the U.S.

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• The source of nuclear energy is the rearrangement
  of protons and neutrons to form more stable
  nuclei.
• In this manner, nuclear reactions are akin to
  chemical reactions the latter occur in nature
  often spontaneously and except for the
  relatively few spontaneous endothermic reactions,
  the products have less internal energy than the
  reactants and can therefore exist in a more
  stable form.

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• Fusion is the combination of light nuclei such as
  the hydrogen nucleus, a single proton, to produce
  something heavier such as the helium nucleus, two
  protons and two neutrons.
• 2H 3H ? 4He 1n
• Fission is the process of breaking apart large
  nuclei such as uranium into some smaller pieces
  such as krypton and iodine.
• Both nuclear reactions function under the
  principle that more stable nuclei result, both
  release very large amounts of energy in the
  process

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Emc2

• 1 amu 4.2 x 10-17 kwh 931 MEV
• The energy evolved from grams of nuclear fuel is
  equivalent to that evolved from tons of chemical
  fuels

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Energy Changes in Nuclear Reactions

• Einstein showed that mass and energy are
  proportional E mc2
• If a system loses mass it loses energy
  (exothermic).
• If a system gains mass it gains energy
  (endothermic).
• Since c2 is a large number (8.99 ? 1016 m2/s2)
  small changes in mass cause large changes in
  energy.
• Mass and energy changed in nuclear reactions are
  much greater than chemical reactions.

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Energy Changes in Nuclear Reactions

• 23892U ? 23490Th 42He
• for 1 mol of the masses are
• 238.0003 g ? 233.9942 g 4.015 g.
• The change in mass during reaction is
• 233.9942 g 4.015 g - 238.0003 g -0.0046 g.
• The process is exothermic because the system has
  lost mass.

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Continued

• To calculate the energy change per mole of 23892U

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Terms

• The 4He particles are known as alpha particles
• High speed electrons emitted by an unstable
  nucleus are known as beta particles
• Gamma radiation consists of high-energy photons
  electromagnetic radiation of very short
  wavelength
• A positron is a particle that has the same mass
  as an electron, but an opposite charge

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Nuclear Fission

• Splitting of heavy nuclei is exothermic for large
  mass numbers.
• Consider a neutron bombarding a 235U nucleus

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Nuclear Fission

• The heavy 235U nucleus can split into many
  different daughter nuclei, e.g.
• 10n 23892U ? 14256Ba 9136Kr 310n
• releases 3.5 ? 10-11 J per 235U nucleus.
• For every 235U fission 2.4 neutrons are produced.
• Each neutron produced can cause the fission of
  another 235U nucleus.

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Nuclear Fission
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Nuclear Fission

• The number of fissions and the energy increase
  rapidly.
• Eventually, a chain reaction forms.
• Without controls, an explosion results.
• Consider the fission of a nucleus that results in
  daughter neutrons.
• Each neutron can cause another fission.
• Eventually, a chain reaction forms.
• A minimum mass of fissionable material is
  required for a chain reaction (or neutrons escape
  before they cause another fission).

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Uranium

• Three naturally occurring isotopes
• Uranium-233
• Uranium 235
• Uranium 238
• Vary in number of neutrons all have 92 protons
• 99.3 is 238U 0.7 235U only trace 233U

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• 235U is the only naturally occurring nuclide that
  is fissionable with thermal neutrons
• Other fissionable materials are 239Pu and 233U
• 235U from uranium ore

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Uranium Ore

• Ore contains 0.05 0.2 U3O8
• U3O8 concentrated from ore by extraction with
  sulfuric acid followed by either ion exchange or
  solvent extraction
• For solvent extraction, typically use organic
  amines to give about 90 U3O8

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UF6

• U3O8 2H2 ? 3 UO2 2 H2O
• UO2 4 HF ? UF4 2 H2O
• UF4 F2 ? UF6 ( distillable)

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UF6

• A mixture of 238U and 235U
• About 0.7 235U
• 235U content is enriched to about 2 4 by a
  variety of techniques typically gaseous
  diffusion through a membrane
• Enriched 235UF6 is hydrolyzed and reduced to
  UO2 ground for preparation of high density
  pellets.

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Nuclear Power Plant
FG21_020.JPG
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• Enriched uranium pellets encased in Zr or
  stainless steel tubes are used for fuel in
  nuclear power plants.
• Control rods made of Cd or B control the fission
  process by absorbing neutrons.
• A moderator slows down the neutrons so they are
  more likely to be captured by the fuel.
• A cooling liquid circulates through the reactor
  and its heat is used to produce steam which
  generates electricity by driving a steam turbine.

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Some Question Nuclear Power Plants, but.
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A Few Other Concepts

• Radioactive decay
• Radon
• Radiotracers
• PET ( Positron Emission Tomography)

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Rates of Radioactive Decay

• 90Sr has a half-life of 28.8 yr. If 10 g of
  sample is present at t 0, then 5.0 g is present
  after 28.8 years, 2.5 g after 57.6 years, etc.
  90Sr decays as follows
• 9038Sr ? 9039Y 0-1e
• Each isotope has a characteristic half-life.
• Half-lives are not affected by temperature,
  pressure or chemical composition.
• Natural radioisotopes tend to have longer
  half-lives than synthetic radioisotopes.

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Rates of Radioactive Decay
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Rates of Radioactive Decay

• Half-lives can range from fractions of a second
  to millions of years.
• Naturally occurring radioisotopes can be used to
  determine how old a sample is.
• This process is radioactive dating.

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Radon

• Radon
• The nucleus 22286Rn is a product of 23892U.
• Radon exposure accounts for more than half the
  360 mrem annual exposure to ionizing radiation.
• Rn is a noble gas so is extremely stable.
• Therefore, it is inhaled and exhaled without any
  chemical reactions occurring.
• The half-life of is 3.82 days.
• It decays as follows
• 22286Rn ? 21884Po 42He

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• The ?-particles produced have a high RBE(Relative
  Biological Effectiveness).
• Therefore, inhaled Rn is thought to cause lung
  cancer.
• The picture is complicated by realizing that
  218Po has a short half-life (3.11 min) also
• 21884Po ? 21482Pb 42He
• The 218Po gets trapped in the lungs where it
  continually produces ?-particles.
• The EPA recommends 222Rn levels in homes to be
  kept below 4 pCi per liter of air.

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Radiotracers

• 131I has been used to test the activity of the
  thyroid gland
• Patient drinks a solution of NaI containing a
  small amount of 131I.
• A Geiger counter determines the ability of the
  thyroid to take up iodine. A normal thyroid will
  absorb about 12 of the iodine within a few hours.

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PET (Positron Emission Tomography)

• Can be used to monitor various compounds such as
  glucose.
• Compound to be detected in the patient must be
  labelled with a radionuclide that is a positron
  emitter.
• Most widely used nuclides are 11C, 18F, 15O, and
  13N.

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• For example, glucose can be labeled with 11C
• Because half life is short, must then quickly
  inject the compound into the patient and then
  place patient in PET instrument

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The End